Various technologies have been proposed for stably producing a grain-oriented electrical steel sheet excellent in magnetic properties having a magnetic flux density B8 (magnetic flux density in magnetic field of 800 A/m) exceeding 1.9 T. The methods of production in the case containing Al as an inhibitor can be classified into the first to third, that is, three types of, technologies shown in Table 1 according to the slab heating temperature.
TABLE 1SlabreheatingSharpness of GossClasstemp.NitridationOrientationRemarkFirstComplete solid solution,≧1300°C.ProhibitedGoodConventionalnonnitridation typemethodSecondSufficient precipitation,<1280°C.EssentialFairnitridation typeThirdPartial precipitation,1200toEssentialIndustrializationnitridation type1350°C.hardComplete solid solution,EssentialGoodlarge nitridation typeFourthComplete solid solution,≧1280°C.EssentialVery goodPresentsmall nitridation typeinvention
The first technology is the complete solid solution non-nitridation type, that is, a method of heating a slab from 1350° C. to an ultra-high temperature of 1450° C. at the highest, then holding the slab at that temperature for a time long enough to uniformly heat (soak) the entire slab. This causes the MnS, AlN, and other substances having inhibitor capabilities to completely dissolve and causes them to function as the inhibitors required for secondary recrystallization. This complete solid-solubilization simultaneously becomes a means for eliminating the difference in inhibitor strength due to the slab position as well and, in this point, is advantageous for realizing stable secondary recrystallization.
In the case of this technology, however, irrespective of the fact that the complete solid-solubilization temperature for securing the amount of inhibitor required for secondary recrystallization is not that high thermodynamically, in actual industrial production, the temperature cannot help becoming an ultra-high temperature in order to secure productivity and a uniform solid solution state of the slab as a whole. Improvement has been attempted, but actual production involves a variety of problems. For example, 1) securing the hot rolling temperature is difficult according to the position and when it cannot be secured, in-slab deviation of the inhibitor strength occurs, therefore poor secondary recrystallization occurs, 2) coarse grains are easily formed at the time of slab heating, the coarse grain parts cannot be secondary recrystallized, and streak-like poor secondary recrystallization occurs, 3) the slab surface layer melts and becomes molten slag and enormous labor becomes necessary for maintenance of the heating furnace, 4) giant edge cracks are easily formed in the steel strip after hot rolling, and so on.
Further, in this technology, as disclosed in ISIJ International, Vol. 43 (2003), No. 3, pp. 400 to 409, Acta Metall., 42 (1994), 2593, KAWASAKI STEEL TECHNICAL REPORT, Vol. 29 (1997)3, 129-135, it is widely known that the Goss orientation sharpness deteriorates when performing nitridation after decarburization annealing up to the start of the secondary recrystallization in order to supplement the inhibitors. Further, it is well known that poor secondary recrystallization occurs when an amount of nitrogen is small at the time of melting.
The second technology is a (sufficient) precipitation nitridation type. As disclosed in Japanese Patent Publication (A) No. 59-56522, Japanese Patent Publication (A) No. 5-112827, Japanese Patent Publication (A) No. 9-118964 etc., this performs the slab heating at a temperature less than 1280° C. and performs the nitridation from after the decarburization annealing to the start of the secondary recrystallization.
In this method, as shown in for example Japanese Patent Publication (A) No. 2-182866, control of the mean grain size of primary recrystallized grains after the decarburization annealing to within a content range, usually a range from 18 to 35 μm, is very important for performing the secondary recrystallization well.
Further, the amount of substances having an inhibitor capability in solid solution in the steel exerts a large influence upon the growth potential of primary recrystallized grains. Therefore, in this technology, in order to make sizes of the primary recrystallized grains in the steel sheet uniform, for example, Japanese Patent Publication (A) No. 5-295443 discloses a method of making the solute nitrogen at the time of the slab heating low to suppress non-uniform precipitation occurring in a later process. From the viewpoint of reduction of the amount of solid solution, the actual slab heating temperature is desirably 1150° C. or less.
In this technology, however, no matter how strictly the chemical compositions are adjusted, the inhibitor substances cannot be left completely coarsely precipitated as they are, so the primary recrystallized grain size tends not to be constant. Therefore, in actual production activities, in order to obtain a suitable primary recrystallized grain size, the conditions of the primary recrystallization annealing (particularly the temperature) are adjusted for each coil. For this reason, the production process becomes troublesome. Further, the formation of the oxide layer in the decarburization annealing is not constant. Therefore, sometimes poor formation of the glass film occurs.
The third technology is the mixed type. As shown in Japanese Patent Publication (A) No. 2000-199015, the slab heating temperature is set to 1200 to 1350° C. and the nitridation is made essential in the same way as the second technology. In order to avoid the ultra-high slab heating temperature exceeding 1350° C. in the first technology, the slab heating temperature is lowered. The insufficient inhibitor strength along with this is made up for by the nitridation. This technology is further classified into two types.
One is the partial solid solution nitridation type (partial precipitation nitridation type), and the other is the complete solid solution nitridation type as represented by Japanese Patent Publication (A) No. 2001-152250. In the former, it is not easy to make the solid solution state industrially uniform in the steel sheet (coil) as a whole. On the other hand, in the latter, the contents of the inhibitor elements are reduced to enable the elements to enter solid solution, therefore a non-uniform state of inhibitors seldom occurs. This is a very logical and effective technology.
This third technology classifies inhibitors into a primary inhibitor for determining the primary recrystallized grain size and a secondary inhibitor for making the secondary recrystallization possible. The primary inhibitor naturally contributes to the secondary recrystallization as well. Due to the presence of the primary inhibitor, the fluctuation in grain size after the primary recrystallization becomes small. Particularly, in the latter complete solid solution type, the primary recrystallized grain size does not change in the usual temperature range, therefore, it is not necessary to change the primary recrystallization annealing conditions for adjustment of the grain size, and the glass film is formed extremely stably.
As the primary inhibitor, the inhibitor substances used in the first technology (for example, AlN, MnS, MnSe, Cu—S, Sn, Sb, etc.) are mainly used. However, to reduce the slab heating temperature, their contents are required to be small. The secondary inhibitor is the AlN which is formed nitrided and these primary inhibitors after the decarburization annealing and up to the start of the secondary recrystallization. Further, the above Japanese Patent Publication (A) No. 2001-152250 also discloses BN as a primary inhibitor. However, N bonds with Al as well, therefore actually sometimes the secondary recrystallization becomes unstable when Al and B are simultaneously contained.
As a problem common to the above three technologies, the fact that the suitable ranges of the contents of the required inhibitor substances (particularly Al and N) are narrower in comparison with the process capability at the time of melting in the steelmaking may be mentioned. Therefore, conventionally, the method of adjusting the production conditions using the acid-soluble Al (hereinafter referred to as “solAl”) minus the N equivalent, that is, AlR, as a parameter is disclosed in the first and second technologies.
In the first technology, for example Japanese Patent Publication (A) No. 60-177131 prescribes adjustment of a soaking time or cooling rate of the annealing before the last cold rolling and/or any of the series of process conditions by the AlR value.
Further, in the second technology, Japanese Patent Publication (A) No. 7-305116 prescribes a ratio of N2 in the atmosphere at the time of the final annealing according to an equation of the AlR. Japanese Patent Publication (A) No. 8-253815 adds Bi and prescribes the temperature of the annealing before the last cold rolling according to the equation of AlR. Japanese Patent Publication (A) No. 8-279408 includes Ti and defines the nitridation amount according to the equation of AlR considering TiN.